【作者】 丁交阳；

【导师】 胡友秋；

【作者基本信息】 中国科学技术大学 ， 空间物理， 2008， 博士

【摘要】 日冕物质抛射(CME)是大尺度的太阳活动现象,它引起地球空间环境的剧烈扰动,是空间灾害天气的主要源头。对CME的观测和理论研究,是太阳物理和日地空间物理领域十分重要的课题。尽管这些研究已经开展了几十年,但是由于观测技术的限制,到现在为止,人们连CME的起源都没有完全研究清楚。本文就CME的起源问题作了相应的研究,并提出了可能的CME触发机制。我们首先回顾了CME的观测特征以及研究现状,然后介绍了CME的理论模型,其中,重点介绍了与本文的工作密切相关的日冕磁绳灾变模型。随后,介绍了我们在日冕磁绳灾变研究方面取得的成果。我们采用球坐标下的2.5维MHD模型,研究了不同背景场中日冕磁绳系统的平衡特性和灾变行为。首先研究了八极子背景场中日冕磁绳系统的灾变行为。背景场包含三个磁拱:两侧的磁拱完全闭合,中心磁拱为部分开放场;磁绳位于中心磁拱之中。初始时磁绳附着于光球表面,系统处于平衡态。我们发现日冕磁绳系统存在灾变行为:当磁绳的环向磁通或者轴向磁通超过某一个临界值时,磁绳向上喷发,逃逸到无穷远。对于无力场情况,我们计算了系统的灾变能阈。我们发现,尽管在所有事例中,灾变能阈都超过了其对应部分开放场的能量,但是灾变能阈的大小与磁绳磁通和中心磁拱的开放程度有关。当中心磁拱的开放磁通给定时,系统的灾变能阈随着灾变点处磁绳环向磁通的增加或者轴向磁通的减小而增加;当灾变点处磁绳的环向磁通和轴向磁通给定时,灾变能阈随着中心磁拱开放磁通的增加而减小。这与偶极场背景下的结果不同。在偶极背景场下,系统的灾变能阈与灾变点处磁绳的磁通和背景场的开放程度几乎没有关系。我们对两种背景场下出现不同结果的原因进行了简要分析。其次,我们研究了八极子背景场中含有多个日冕磁绳系统的灾变行为。背景磁场为八极子无力场,它包含三个双极场,位于中心的双极场部分开放。每个双极场内部各有一根磁绳,它们均附着于光球表面。从这个初态出发,我们通过增加任一个磁绳的环向磁通或者轴向磁通,在理想MHD和电阻MHD两种情况下分别分析系统的灾变行为。结果表明,当任一个磁绳的环向或者轴向磁通超过某个临界值时,该磁绳将向上喷发,在其下面生成一个新的电流片。其它的磁绳的命运取决于新生电流片上是否发生磁场重联。在理想MHD即不出现重联的情况下,其余磁绳将依然附着于光球表面,处于平衡状态;而在电阻MHD的情况下,这些磁绳也会向上喷发。在此过程中,磁场重联扮演了十分重要的角色,它引起背景磁场拓扑位形的改变,减弱了背景场对依然附着于光球的磁绳的约束,降低了相应的灾变能阈,导致这些磁绳的灾变式喷发。含有多个日冕磁绳系统的灾变行为表明,位于不同活动区的独立磁通量系统之间的相互作用提供了一个可能的物理机制去解释观测到的同调事件。最后,我们研究了背景场存在新磁通浮现的情况下,日冕磁绳系统的灾变行为,以此来重新考察新磁通浮现与CME的关系。在本研究中,我们固定磁绳的磁通不变,通过引入光球磁通浮现来改变背景场,从中考察日冕磁绳系统对磁通浮现的相应。初始时背景场为无力场,磁绳附着于光球表面,外部为双极场。在光球表面的某个区域浮现新的磁通后,背景磁场的拓扑位形将会发生改变,在新浮现磁通和背景磁通之间会形成电流片。我们发现,随着浮现磁通量的增加,日冕磁绳系统在有些情况下会存在灾变行为,其影响因素包括新浮现磁通的位置、方向以及新浮现磁通与背景磁通之间的电流片上面是否发生磁场重联。对于浮现区位于赤道一侧的情况,我们按浮现磁通取向分为两类。对于浮现磁通与背景磁通同向的情形,不管新生电流片上是否发生磁场重联,只要新浮现磁通超过某个临界值,磁绳就会向上喷发,发生灾变。新生电流片上的重联的作用仅仅在于降低了上述浮现磁通的临界值。相反,如果浮现磁通与背景磁通反向,则磁绳始终附着于光球,不会发生灾变。当浮现区中心位于赤道时,浮现磁通和背景磁通之间形成的电流片上是否存在磁场重联对系统的灾变行为有着重要的影响。若不存在磁场重联,不管浮现磁通的取向如何,系统都存在灾变,只不过当浮现磁通与背景磁通反向时灾变更容易被触发。当新生电流片上出现磁场重联时,只有当浮现磁通与背景磁通反向时,系统才会发生灾变;与不出现重联的情况相比,系统灾变的触发变得更加容易。当考虑磁场重联时,上述关于新磁通浮现与CMEs之间的关系的结论表面上与前人的结论一致,但我们是从MHD灾变观点出发而获得这些结论的。在我们的研究中,新磁通浮现对磁绳的喷发起的作用是触发,而非驱动。“慢”的磁通浮现逐渐改变背景磁场的拓扑位形,从而影响系统的灾变行为。在合适情况下,磁通浮现会使系统状态靠近并最终抵达灾变点,导致灾变发生和磁绳的突然“快速”向上喷发。更多还原

【Abstract】 Coronal mass ejections (CMEs) belong to large-scale solar active phenomena. They cause strong disturbances of the Earth’s space environment, and serve as a main source of disastrous space weather. The observational and theoretical study of CMEs has been as an important subject in the fields of solar physics and solar-terrestrial space physics. Although the study has been carried out for several decades, the origin of CMEs is still not fully understood because of the difficulty in observing them directly. In this thesis, we study the origin of CMEs and propose some possible triggering mechanisms for CMEs.After a brief review of observational features and current research status of CMEs, we summarize the theoretical models of CMEs, especially the coronal flux rope catastrophe model closely related to this thesis. Then we describe our main results in the study of coronal flux rope catastrophe. In terms of a 2.5-dimensional MHD model in spherical geometry, we investigate the equilibrium properties and catastrophic behaviors of the coronal flux rope system in different background magnetic fields with complicated topology.First, we study the catastrophic behavior of the coronal flux rope system in an octapole background magnetic field that contains three magnetic arcades: a partly open one in the center across the equator and two fully closed in the flank, and a magnetic flux rope inside the central arcade. The initial state is in equilibrium and the flux rope is attached to the solar surface. With respect to an increase of either the annular flux or the axial flux of the rope, the flux rope system exhibits a catastrophic behavior: the rope erupts upward and escapes to infinity as either of the two fluxes exceeds a certain critical value. Under the force-free field regime, we calculate the catastrophic energy threshold of the system. We find that the threshold depends on the magnetic fluxes of the flux rope and the extent to which the central arcade is open, though the catastrophic energy threshold is larger than that of the corresponding partly open field in all cases. For a given open flux of the central arcade, the energy threshold increases with increasing annual flux or decreasing axial flux of the flux rope. Moreover, for given annular and axial fluxes of the flux rope, the more open the central arcade is, the lower of the threshold will be. These results differ from those for the bipolar background field case, in which the catastrophic energy threshold is almost independent of the magnetic fluxes of the flux rope and the extent to which the background field is open. The reason for such a difference is briefly discussed.Secondly, we study the catastrophic behavior of multiple coronal flux rope system. The background magnetic field is an octapole force-free field containing three bipolar fields, and the central bipolar component is partly opened. A flux rope is introduced within each bipolar field. Starting from this state, we increase either the annular or the axial flux of a certain flux rope to examine the catastrophic behavior of the system in two regimes, the ideal MHD regime and the resistive MHD regime. It is shown that when the annular flux or the axial flux of the rope of interest exceeds a certain critical value, the rope breaks away from the base and escapes to infinity, leaving a current sheet below. The destiny of the remainder flux ropes relies on whether magnetic reconnection takes place across the newly formed current sheet. In the ideal MHD regime, i.e., in the absence of reconnection, these ropes remain to be attached to the solar surface in equilibrium, whereas in the resistive MHD regime they also erupt upward. During this process, magnetic reconnection plays a crucial role: it changes the topology of the background field outside the attached flux rope in such a way that the constraint on these ropes is substantially relaxed and the corresponding catastrophic energy threshold is reduced accordingly, leading to a catastrophic eruption of these ropes. The catastrophic behavior of multiple coronal flux rope system demonstrates that the interaction between several independent magnetic flux systems in different active regions provides a possible mechanism for sympathetic events occurring on the sun.Finally, we study the catastrophic behavior of coronal flux rope system caused by photospheric flux emergence somewhere on the photosphere in or- der to reexamine the relationship between photospheric magnetic flux emergence and solar explosive phenomena such as CMEs. In this study, we fix the magnetic fluxes of the flux rope, introduce a photospheric flux emergence to change the background field, and examine how the flux rope system responds to the flux emergence. The initial magnetic field is taken to be a force-free field, consisting of an isolated flux rope attached to the base and a bipolar background field surrounding it. A flux emergence is then introduced somewhere on the photosphere, and it causes a variation of the topology of the background field and a formation of a current sheet at the interface between the newly emerging and preexisting fluxes. It is shown that as the total emerging flux increases, the flux rope system exhibits a catastrophic behavior in some cases, and the relevant controlling factors include the location and field orientation of the newly emerging arcade and whether magnetic reconnection takes place across the newly formed current sheet. For the situation that the emergence region lies away from the equator, we divide it into two categories according to the orientation of the emerging flux. For the case that the emerging flux and the background flux are the same in orientation, the flux rope erupts upward and a catastrophe takes place provided that the emerging flux exceeds a certain critical value irrespective as to whether magnetic reconnection occurs across the newly formed current sheet. The role of the reconnection across the newly formed current sheet is just to reduce the critical value of the emerging flux mentioned above. On the contrary, if the emerging flux and the background flux are opposite in orientation, the flux rope always sticks to the photosphere, and no catastrophe occurs. If the center of the emergence region is located at the equator, the existence of magnetic reconnection across the current sheet formed between the emerging flux and the background flux does play an important role in the catastrophic behavior of the system. In the absence of magnetic reconnection, catastrophe exists for the system irrespective the orientation of the emerging flux. Nevertheless, it becomes easier for a catastrophe to be triggered if the orientation of the emerging flux is opposite to that of the background flux. In the presence of magnetic reconnection, only when the orientation of the emerging flux is opposite can a catastrophe be present, and moreover, in comparison with the case without magnetic reconnection, the triggering of the catastrophe of the system becomes easier. The conclusions in the presence of magnetic reconnection described above are superficially similar to those reached by previous studies in regard to the relationship between photospheric new flux emergence and coronal flux rope eruption. However, we have reached these conclusions based on the viewpoint of MHD catastrophe. In this context, the role of the flux emergence in the flux rope eruption is not a driver but a trigger. "Slow" photospheric flux emergence changes the topology of the background field gradually so as to affect the catastrophic behavior of the whole system. In appropriate cases, the flux emergence causes the state of the system to near and eventually to reach its catastrophic point, leading to the occurrence of a catastrophe and an abrupt "fast" eruption of the flux rope.更多还原